In the previous phases of this study, embryonic cardiac output was shown to be a function of embryonic blood volume, and blood volume was shown to be affected by alterations in the hydrostatic and colloid osmotic pressures acting in the circulation. Continuing work is directed towards the relation between embryonic cardiac ultra structure and contractility and to the identification of the extra embryonic compartment against which the embryo plasma equilibrates when its hydrostatic and osmotic pressures are altered.